1. Field of the Invention
The present invention relates to stable solidification and curing accelerators for hydraulic binders in dissolved form, and to their preparation and use.
2. Description of the Related Art
EP 0 76 927 B1 discloses an alkali metal-free setting accelerator based on aluminium hydroxide. The further constituents of this composition are, according to Claim 1, at least one water-soluble sulphate and/or nitrate and/or formate of alkaline earth metals and transition metals which are added to the main constituent by simple mixing. It is unambiguously stated on page 2, line 55, that this composition is a pulverulent mixture which is added in solid form to the hydraulic binder. Water-swellable compounds are added to the mixture only to avoid excessive dust formation.
DD 266 344 A1 describes cationic aluminium salts obtained by reacting a precipitated aluminium hydroxide with concentrated formic, acetic or nitric acid, which salts are mixed in the form of a powder into the hydraulically setting cement or mortar concerned. The compounds formed by reaction of the aluminium hydroxide with the appropriate acids can be described by a general formula, e.g. Al (OH).sub.2 R.times.H.sub.2 O, Al (OH) R.sub.2.times.H.sub.2 O or AlR.sub.3.times.H.sub.2 O. These aluminium salts contain no sulphate or chloride ions.
FR 2 471 955 describes a process for the accelerated setting of cement which is characterized in that bisulphite-formaldehyde, calcium formate and calcium nitrate in the form of a solution are used as accelerating agent, e.g. see page 3, from line 19, Examples.
Finally, EP 0 657 398 A1 likewise discloses a process for accelerating the setting of hydraulic binders, which is characterized in that the accelerating agent is a mixture containing at least one water-soluble sulphate of a polyvalent cation, in particular of aluminium, and a further water-insoluble component, e.g. calcium sulphoaluminate, calcium aluminate, basic aluminium sulphate and at least one further component. This further component can, according to Claim 24, be a dispersant, an inorganic binder, a fluidizer or a deaerating agent.
The setting accelerators are of particular interest in industry if they can be used in liquid form. Compared with the solid accelerators, mention may be made of essentially their better meterability, the more rapid release of the active substance (s) and naturally the negligible dust or aerosol formation during handling.
The classical liquid, alkaline accelerators, e.g. sodium aluminate and water glass are not only corrosive because of their high pH but also lead to a sometimes drastic decrease in the final strength of concrete because of the high amounts of alkali metal which they introduce.
Newer, liquid accelerators comprise mainly aqueous suspensions of basic aluminium sulphates. However, they are generally not stable to freezing, tend to sediment and display, although they could be employed in liquid form in the wet spraying process, unsatisfactory early strength development and also have only a slight accelerating effect, depending on the type of cement. Other commercially available concentrated aluminium salt solutions, e.g. Al.sub.2 (SO.sub.4).sub.3 containing about 8% of Al.sub.2 O.sub.3 or aluminium triformate solutions containing about 5% of Al.sub.2 O.sub.3, are unsuitable, at least for the wet spraying process, since they result in an undesirably low compressive strength because of the increased amount of water introduced into the concrete composition and the associated increased water-cement factor.
It is therefore an object of the present invention to provide a concentrated solidification and curing accelerator in aqueous solution form for hydraulic binders, which accelerator does not have the disadvantages of the classical liquid accelerators such as sodium aluminate and water glass and is, in particular, well suited to the wet spraying process.